Some computer networks, such as local area networks (LANs) or intranets, employ a lower level of security against malicious attacks when local computers within a computer network access documents and resources from within the same computer network than when they access documents and resources via external computer networks. This unfortunately makes such computer networks vulnerable to attack. For example, a computer user within a local computer network may unwittingly access a web page retrieved from an external computer network that attempts to exploit resources at network addresses within the local computer network, leveraging the fact that default security settings, such as of IP addresses, port numbers, and credentials, are often left unchanged. These attacks can be used to spread malware, access sensitive information, and penetrate further into the attacked network.